1. Field of the Invention
The present invention relates to an apparatus and method for reducing the recovery period of a probe in nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) detection systems, and, more particularly, to an apparatus and method for reducing the recovery period by varying the impedance of a load to lower the total Q factor of the probe and load during the recovery period as compared to the total Q factor during transmission and reception.
2. Description of the Related Art
There are many situations where it is desirable to detect the presence of a specific substance. For example, with the unfortunate increase in drug trafficking and terrorist use of high explosives in aircraft and buildings, there is a strong interest for a reliable detection system that can detect sub-kilogram quantities of narcotics and explosives against a background of more benign materials in a rapid, accurate, and non-invasive fashion.
Nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) are known techniques for detecting the presence of specific substances. More specifically, various substances produce a magnetic resonance signal when excited by radio frequency (RF) radiation at a particular frequency. Generally, RF radiation at a particular frequency will cause a precession of nuclei in a specific substance, but not in other substances. Nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) take advantage of this phenomenon to detect the various substances (for NMR a magnet is included but not illustrated).
FIG. 1 illustrates a conventional NQR and NMR apparatus. As illustrated in FIG. 1, a transmitter 20 and a receiver 22 are connected to a probe 24 through a transmit/receive (T/R) switch 26. Probe 24 includes a coil (not illustrated) forming part of a resonant circuit (not illustrated). To detect the presence of a specific substance, T/R switch 26 connects transmitter 20 to probe 24 while disconnecting receiver 22 from probe 24. Then, transmitter 20 generates a pulse and supplies the pulse to probe 24. Generally, the pulse is formed from a signal having a frequency corresponding to the resonance frequency of the nuclei of the specific substance that is intended to be detected. The pulse is transmitted to probe 24, which causes the coil in probe 24 to store RF energy and generate an RF magnetic field at a target specimen (not illustrated). If the specific substance desired to be detected is present in the target specimen, the RF magnetic field generated by the coil will excite nuclear resonance in the quadrupolar nuclei of the specific substance and thereby cause the specific substance to produce a resonance signal.
After the RF magnetic field is generated by the coil of probe 24, the T/R switch 26 connects receiver 22 to probe 24 while disconnecting transmitter 20 from probe 24. The coil in probe 24 then detects the resonance signal produced by the specific substance, and probe 24 produces a corresponding output signal. The output signal of probe 24 is received and analyzed by receiver 22, to confirm the presence of the specific substance in the target specimen.
Therefore, probe 24 generates an RF magnetic field at a specimen, and also receives resonance signals produced by the specific substance in the specimen. However, probe 24 cannot receive the resonance signal immediately after generating an RF magnetic field, because of the stored RF energy in the probe. Instead, immediately after the excitation pulse, probe 24 must "ring down", i.e., dissipate the stored RF energy, before it can usefully receive the resonance signal.
FIG. 2(A) illustrates a pulse 25 produced by transmitter 20 and provided to the coil of probe 24 from time t.sub.1 to t.sub.2, and FIG. 2(B) illustrates the corresponding RF energy 27 stored in the coil. As illustrated by FIGS. 2(A) and 2(B), some RF energy still remains in the coil after the end of pulse 25 at time t.sub.2 until time t.sub.3. This remaining RF energy must be dissipated, that is, probe 24 must "ring down", before probe 24 can effectively be used to receive a resonance signal. The time required for probe 24 to "ring down" to an appropriate level is referred to as a "recovery period", and is illustrated in FIG. 2(B) as recovery period R.
Unfortunately, a long recovery period will undesireably decrease the detection sensitivity of probe 24, because of the loss of the resonance signal. For example, if the lifetime of a resonance signal is shorter than, or comparable to, the recovery period of the probe, then the resonance signal will have decayed substantially before it can be detected.
For a single coil employed in probe 24, a rough rule of thumb is that a useful resonance signal cannot be received by probe 24 until approximately twenty (20) time constants have elapsed from the end of the RF pulse provided to the coil of probe 24. The time constant of the coil is given by Q/.pi.f, where f is the resonance frequency (in Hz), and Q is the quality factor of the coil. Therefore, the recovery period typically must be at least 20Q/.pi.f. This recovery period becomes particularly long with NQR and NMR operating at relatively low frequencies, such as at frequencies less than 10 MHz and for high-Q coils.
Moreover, the signal-to-noise ratio (SNR) of a resonance signal received by the coil is increased when the coil has a high Q. Therefore, it is desirable to have a coil with a high Q. However, as can be seen from the time constant Q/.pi.f, the recovery period can be relatively long for high Q coils. As a result, using the prior art, a system designer must compromise between the desire to have a high Q coil, and the desire to reduce the recovery period. For example, a coil with a Q of 1,000 at a frequency of 1 MHz has an estimated recovery period of 6 ms. This recovery period is too long to reliably detect many substances, and the Q of the coil may have to be reduced.
In addition, in NQR and NMR, a train of sequential pulses is typically generated by transmitter 20 and supplied to probe 24 for generating a series of RF magnetic fields at a specimen. After each pulse is provided to the coil in probe 24 and the corresponding RF magnetic field is generated, probe 24 must wait for a respective recovery period to elapse. After the recovery period elapses, probe 24 can receive resonance signals. Further, after a period of time elapses for receiving resonance signals, the next pulse is generated. Therefore, a recovery period must pass between each pulse. This recovery period dictates the time scale for which the train of RF pulses can be applied.
For example, in a conventional steady-state free-precession sequence of RF pulses, the pulses are spaced at a time interval .tau.. Under certain conditions, it is observed that the amplitude of the resonance signal received in each time interval .tau. desirably increases as the time interval .tau. decreases. Moreover, a resonance signal produced by a specimen can be sampled more frequently by decreasing the time interval .tau., thereby increasing the overall signal-to-noise ratio (SNR). In addition, for methods employing stochastic excitation, the excitation bandwidth is limited by the time interval .tau.. Therefore, a relatively short time interval .tau. is generally preferred. Unfortunately, a long recovery period in the probe necessarily increases .tau..
Conventional methods have been used to "actively damp" the probe, and thereby reduce the recovery period. For example, the recovery period can be actively damped by switching the total Q factor back and forth from a high total Q factor to a low total Q factor, where the total Q factor is switched low during the recovery period. This low total Q factor during the recovery period reduces the recovery period. However, such methods modify the circuitry of the probe to switch the total Q factor. In other words, the electrical configuration of the probe is changed to switch the total Q factor. Such changing of the electrical configuration of the probe can result in a loss of detection sensitivity of the probe.
Moreover, conventional active damping methods generally induce switching transients which cause the resonant circuit of the probe to ring anew. Also, resistive elements in the probe or the active damping circuitry can reduce the signal-to-noise ratio. In addition, some conventional active damping methods are too complex for general use and generally require a system operator to balance several bridge circuits, or balance positive and negative voltage limiters on amplifier feedback circuits. Such circuitry does not provide the efficiency and reliability required for most commercial applications.